Dehydration processes using membranes with hydrophobic coating

ABSTRACT

Processes for removing water from organic compounds, especially polar compounds such as alcohols. The processes include a membrane-based dehydration step, using a membrane that has a dioxole-based polymer selective layer or the like and a hydrophilic selective layer, and can operate even when the stream to be treated has a high water content, such as 10 wt % or more. The processes are particularly useful for dehydrating ethanol.

RELATED APPLICATIONS

This application is a divisional and claims the benefit of U.S. Ser. No.11/897,675 filed Aug. 30, 2007, the disclosure of which are herebyincorporated herein by reference in their entireties.

This invention was made in part with Government support under awardnumber NRCS-68-3A75-4-140, awarded by the United States Department ofAgriculture. The Government has certain rights in this invention.

FIELD OF THE INVENTION

The invention relates to dehydration processes, particularlymembrane-based dehydration processes.

BACKGROUND OF THE INVENTION

Membrane-based processes for removing water from water/organic compoundmixtures are known. The processes may use membranes that are selectivein favor of water over organics, or selective for organics over water.Most processes use water-selective membranes. Such membranes typicallyhave a dense, hydrophilic water-selective layer on a porous support, andare very effective at treating solutions or mixtures in which water ispresent in relatively small amounts.

Representative membranes of this type are described in U.S. Pat. No.4,755,299, for example. Typical materials from which the selective layerof the membrane are made include polyvinyl alcohol (PVA), cellulosetriacetate and other cellulose derivatives. If large amounts of waterare present, this selective layer tends to swell, reducing theseparation capability of the membrane. Under prolonged exposure to highwater concentrations, the membrane may start to dissolve or disintegratecompletely. The problem is exacerbated if the feed solution is hot. Suchmembranes cannot be used to treat fluids that have high waterconcentrations.

In a variety of industrial applications, most notably manufacture ofalcohols from biomass, the solutions to be treated are hot and cancontain 20, 30, 40, 50 wt % or more of water. To treat these and othersimilar solutions or vapor mixtures, there remains a need for membranesthat are able to withstand such conditions.

U.S. Published Patent Application 2007/0031954, co-owned with thepresent application, describes an ethanol recovery process using bothethanol-selective and water-selective membranes.

U.S. patent application Ser. No. 11/715,245, entitled Liquid-Phase andVapor-Phase Dehydration of Organic/Water Solutions, and co-owned withthe present application, describes the use of fluorinated dioxolemembranes to dehydrate alcohols and other organic compounds. This patentapplication is incorporated herein by reference in its entirety.

U.S. Pat. No. 6,896,717 describes gas separation membranes havinghydrocarbon-resistant coatings, include those made from perfluorinatedpolymers of various types, to protect them from attack by C₃₊hydrocarbons in the feed stream.

SUMMARY OF THE INVENTION

The invention is a process for removing water from mixtures of waterwith organic compounds. The mixture may be a solution or a vapor-phasemixture. If the mixture is a solution, a basic embodiment of theprocesses includes the following steps:

(a) providing a composite membrane having a feed side and a permeateside, the composite membrane comprising:

(i) a microporous support layer;

(ii) a first dense selective layer of a hydrophilic polymer; and

(iii) a second dense selective layer of a dioxole-based polymer havingthe structure

wherein R₁ and R₂ are fluorine or CF₃, R₃ is fluorine or —O—CF₃, and xand y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1;the first dense selective layer being positioned between the microporoussupport layer and the second dense selective layer;(b) passing a feed solution comprising water and an organic compoundacross the feed side;(c) withdrawing from the feed side a dehydrated solution having a lowerwater content than that of the feed solution;(d) withdrawing from the permeate side a permeate vapor having a higherwater content than that of the feed solution.

If the mixture is in the vapor phase, a basic embodiment of the processincludes the following steps:

(a) providing a composite membrane having a feed side and a permeateside, the membrane comprising:

(i) a microporous support layer;

(ii) a first dense selective layer of a hydrophilic polymer; and

(iii) a second dense selective layer of a dioxole-based polymer havingthe structure

wherein R₁ and R₂ are fluorine or CF₃, R₃ is fluorine or —CF₃, and x andy represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1;the first dense selective layer being positioned between the microporoussupport layer and the second dense selective layer,(b) passing a feed vapor comprising water and an organic compound acrossthe feed side;(c) withdrawing from the feed side a dehydrated vapor having a watercontent lower than that of the feed solution;(d) withdrawing from the permeate side a permeate vapor having a higherwater content than the feed solution.

In both basic embodiments, the driving force for transmembranepermeation of water is the difference between the vapor pressure ofwater on the feed and permeate sides of the membrane. This pressuredifference can be generated in a variety of ways, for example, byheating the feed liquid, compressing the feed vapor and/or maintaininglower pressure or a partial vacuum on the permeate side.

In the first embodiment, the process is carried out under pervaporationconditions. By pervaporation conditions, we mean that the feed is in theliquid phase, and the pressure on the permeate side is such that thepermeating water is in the gas phase as it emerges from the membrane.The process results, therefore, in a permeate vapor stream enriched inwater, and a liquid residue stream depleted in water.

In the second embodiment, both the feed and permeate streams are in thevapor phase. The process results in a permeate vapor stream enriched inwater, and a vapor residue stream depleted in water.

In both cases, the composite membrane has at least three layers: amicroporous support layer, a thin, dense hydrophilic layer on themicroporous support, and a thin, dense dioxole-based layer on thehydrophilic layer. Representative polymers that can be used for thehydrophilic layer include polyvinyl alcohol (PVA); cellulose acetate,and other cellulose derivatives; polyvinyl pyrrolidone (PVP);ion-exchange polymers, such as Nafion® and other sulfonated materials;and chitosan.

The dioxole-based layer is made from specific dioxole-based polymers,preferably either having the structure:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1, or the structure

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.

Unexpectedly, we found in earlier work that these and other hydrophobic,dioxole-based polymers can be used effectively as selective layers ofcomposite membranes to separate water from organic compounds, even polarorganic compounds. The use of these and other similar polymers asselective layers for dehydration membranes and processes is disclosed inU.S. Pending application Ser. No. 11/715,245.

In the present invention, both the hydrophilic layer and thedioxole-based layer have selectivity for water over the organiccompounds from which the water is to be removed. The intrinsicselectivity of the hydrophilic polymer is normally higher than that ofthe dioxole-based polymer.

Very surprisingly, we have found that, when membranes having the abovestructures are used, the processes of the invention can manifest higherselectivity for water over the organic compound than can be achievedunder the same process conditions by either the hydrophilic polymer orthe top layer polymer used alone as the selective layer of the membrane.

The feed fluid to be treated by the process of the invention contains atleast water and an organic compound. The water may be a minor componentor the major component of the fluid, and can be present in anyconcentration. The fluid may be a solution or a vapor-phase mixture.

The organic compound may be any compound or compounds able to formsolutions or vapor mixtures with water. Our processes are particularlyuseful for removing water from polar organic compounds, such as ethanoland other alcohols, and other organic compounds in which water isreadily soluble or miscible with water, such as esters or organic acids.Such separations are important in the manufacture of bioethanol andother biofuels.

In either embodiment, the process may be configured in various ways, andmay include a single membrane unit or an array of two or more units inseries or cascade arrangements, as is familiar to those of skill in theart.

In another aspect, the invention is a composite membrane comprising:

(i) a microporous support layer;

(ii) a first dense selective layer of a hydrophilic polymer, and

(iii) a second dense selective layer of a dioxole-based polymer havingthe structure

wherein R₁ and R₂ are fluorine or CF₃, R₃ is fluorine or —O—CF₃, and xand y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1;the first dense selective layer being positioned between the microporoussupport layer and the second dense selective layer.

The membrane is preferentially characterized in that, when challengedwith a feed solution containing 20 wt % water at a set of operatingconditions that include a temperature of 75° C., the composite membranehas a higher water/organic compound selectivity than that of either (a)a first membrane having only a hydrophilic polymer selective layer ofthe same hydrophilic polymer as the first dense selective layer, or (b)a second membrane having only a dioxole-based polymer selective layer ofthe same dioxole-based polymer as the second dense selective layer, allas measured at the set of operating conditions.

The processes of the invention may include additional separation steps,carried out, for example, by adsorption, absorption, distillation,condensation or other types of membrane separation. One preferredembodiment of the invention of this type comprises a stripping ordistillation step followed by a membrane separation step carried outusing multilayer composite membranes as described above.

In another aspect, the invention is a process for making ethanol bycombining a fermentation step, with multiple water/ethanol separationsteps in series, one of the separation steps being a membranedehydration step carried out using multilayer composite membranes asdescribed above.

In all aspects of the invention, another, but less preferred alternativeis to use another type of perfluorinated, high permeability material forthe second selective layer.

It is to be understood that the above summary and the following detaileddescription are intended to explain and illustrate the invention withoutrestricting its scope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing of a basic embodiment of the invention.

FIG. 2 is a schematic drawing of a basic pervaporation embodiment of theinvention.

FIG. 3 is a schematic drawing of a basic vapor phase embodiment of theinvention, including compression of the feed vapor.

FIG. 4 is a schematic drawing of an embodiment of the invention in whichthe membrane separation is combined with separation by stripping ordistillation.

FIG. 5 is a schematic drawing of a process for producing alcohol frombiomass.

FIG. 6 is a schematic drawing of an embodiment of the invention in whichthe membrane separation is combined with stripping, and in which themembrane separation is performed as a two-step process.

FIG. 7 is a schematic drawing of a process for producing alcohol frombiomass, in which the membrane separation is performed as a two-stepprocess.

FIG. 8 is a graph comparing the performance of Hyflon® AD, Teflon® AFand Celfa CMC VP-31 membranes in the form of a plot of permeate waterconcentration against feed water concentration at different feed waterconcentrations.

FIG. 9 is a plot comparing the water permeances of Celfa CMC VP-31membranes having Hyflon® AD layers of different thicknesses.

FIG. 10 is a graph comparing the water/ethanol selectivity of Celfa CMCVP-31 membranes, Hyflon® AD membranes and Celfa CMC VP-31/Hyflon® ADmembranes at different feed water concentrations.

FIG. 11 is a graph comparing water permeance of otherwise similarmembranes having Hyflon® AD and Teflon® AF layers.

DETAILED DESCRIPTION OF THE INVENTION

The term mixture as used herein means any combination of an organiccompound and water, including solutions and vapor-phase mixtures. Theterm also refers to a solution, plus undissolved organics or waterpresent as a separate phase. As used herein, the term mixture typicallyrefers to mixtures of an organic compound and water that are liquid atroom temperature and pressure.

The term separation factor refers to the overall separation factorachieved by the process. The separation factor is equal to the productof the separation achieved by evaporation of the liquid and theselectively achieved by selective permeation through the membrane.

All liquid mixture percentages herein are by weight unless otherwisestated.

Gas or vapor mixture percentages are by volume unless otherwise stated.

The invention is a process for removing water from fluid mixturescontaining water and organic compounds. The fluid may be in the gas orthe liquid phase.

The separation is carried out by running a liquid or vapor stream of thefluid mixture across a membrane that is selective for water over theorganic compound. The process results in a permeate stream enriched inwater and a residue stream depleted of water, that is, dehydrated.

In one embodiment, the process is performed under pervaporationconditions, as explained in more detail below, so that the feed is inthe liquid phase and the permeate stream is in the gas or vapor phase.

In another embodiment, the process is performed in the gas or vaporphase so that the feed and permeate streams are both in the gas or vaporphase.

The process of the invention can be used to dehydrate many water/organicmixtures. We believe the process of the invention is of particular valuein dehydrating solutions or vapor mixtures containing an organiccompound that has good mutual miscibility or solubility with water,especially those containing an organic compound in which water has asolubility of at least about 5 wt % or 10 wt %. By way of example, theprocess of the invention is particularly useful for separating waterfrom alcohols, ketones, aldehydes, organic acids and esters, includingmethanol, ethanol, isopropanol, butanol, acetone, acetic acid andformaldehyde.

One or multiple organic compounds may be present in the solution to bedehydrated. A common example of a multi-organic mixture to be treated isABE, an acetone-butanol-ethanol mixture typically produced byfermentation and used as a source of butanol and other valuablechemicals.

The processes of the invention are characterized in terms of thematerials used for the selective layers of the membrane, or by theprocess operating conditions in terms of water concentration in the feedmixture.

The streams to which the present invention applies are predominantlycomposed of organic components and water; however inorganic components,including salts or dissolved gases, may be present in minor amounts.

Water may be a major or minor component of the mixture, and the waterconcentration may range from ppm levels to 80 wt % or more, for example.Unlike most prior art membrane dehydration processes, the process issuitable for streams containing large amounts of water, by which we meanstream containing more than about 10 wt % water, and in particularstreams containing more than about 15 wt %, 20 wt %, 30 wt % water, oreven streams in which water is the major component.

The scope of the invention is not limited to any particular type ofstream. The feed streams may arise from diverse sources that include,but are not limited to, fermentation processes, chemical manufacturing,pharmaceutical manufacturing, electronic components manufacture, partscleaning, processing of foodstuffs and the like. As a particularexample, the invention is useful for separating ethanol and water from afermentation broth arising from bioethanol production.

The process of the invention in various embodiments is shown in FIGS.1-5. It will be appreciated by those of skill in the art that these arevery simple schematic diagrams, intended to make clear the key aspectsof the invention, and that an actual process train will usually includemany additional components of a standard type, such as heaters,chillers, condensers, pumps, blowers, other types of separation and/orfractionation equipment, valves, switches, controllers, pressure-,temperature-, level- and flow-measuring devices and the like.

A simple flow diagram of a basic embodiment of the invention is shown inFIG. 1. Referring to this figure, feed stream, 107, enters membraneunit, 100, and flows across the feed side, 105, of composite membrane,101. The membrane has three layers: a microporous support membrane, 102,a hydrophilic layer, 103, and a dioxole-based layer, 104. Thehydrophilic layer is positioned between the support layer and thedioxole-based layer. These three layers are now discussed individually.

So long as it offers essentially no resistance to permeation comparedwith the selective layers, the nature of the support membrane is notcritical to the invention, and the membrane may be made from suchtypical known materials as polysulfone, polyetherimide (PEI),polyacrylonitrile, and polyvinylidene fluoride (PVDF), for example. Themost preferred support layers are those with an asymmetric structure,having a smooth, comparatively dense surface on which to coat theselective layer. Optionally and preferably, the support membraneincludes a porous backing web, not shown, onto which the supportmembrane has been solution-cast.

The hydrophilic polymer layer is adjacent to the support membrane, andthe dioxole-based polymer layer is the top selective layer. The layersoperate together to provide properties that could not be provided byeither layer alone. Both layers are made from polymers that have highwater/organic compound selectivity, at least when tested with solutionsthat contain no more than about 10 wt % water. The hydrophilic polymerhas higher intrinsic selectivity than the dioxole-based polymer, andpreferably should have a selectivity of at least about 200 under lowwater concentration test conditions (less than 10 wt % water). Suitablehydrophilic polymers include, but are not limited to, polyvinyl alcohol(PVA); cellulose acetate and all other cellulose derivatives, polyvinylpyrrolidone (PVP); ion-exchange polymers, such as Nafion® and othersulfonated materials; and chitosan.

The top selective layer polymer is a dioxole-based polymer. Thesepolymers are hydrophobic, and generally exhibit much lower waterpermeability and water/organic compound selectivity than hydrophilicpolymers membranes under low water concentration test conditions (lessthan 10 wt % water). Despite their hydrophobic nature, however, wepreviously discovered that membranes formed from these polymers canoperate well to dehydrate organic/water solutions. Unlike theirhydrophilic counterparts, they can maintain a relatively stableperformance when exposed to fluid mixtures with high waterconcentrations, such as more than 20 wt % water, even when the mixtureis hot.

A measure of the chemical stability and hydrophobic nature of thepolymer is its resistance to swelling when exposed to water. This may bemeasured in a very simple manner by weighing a film of the pure polymer,then immersing the film in boiling water for a period. When the film isremoved from the water, it is weighed immediately, and again after thefilm has been allowed to dry out and reach a stable weight.

The dioxole-based polymer that forms the top selective layer of ourmembrane is sufficiently stable in the presence of water that a film ofthe polymer immersed in water at 100° C. for 24 hours at atmosphericpressure will experience a weight change of no more than about 10 wt %,and more preferably no more than about 5 wt %. If the film is removedfrom the boiling water and weighed immediately, its weight will haveincreased compared with the original weight because of the presence ofsorbed water. This weight increase should be no more than 10 wt % andpreferably no more than 5 wt %. After the film is dried out and theweight has stabilized, it is weighed again. If the film has suffereddegradation as a result of the water exposure test, the weight may havedecreased. The weight loss compared with the original weight should beno more than 10 wt % and preferably no more than 5 wt %.

In contrast, the polymer used for the hydrophilic layer almost alwaysfails this test.

The preferred dioxole-based polymers for use in the present inventionare copolymers having the structure

wherein R₁ and R₂ are fluorine or CF₃, R₃ is fluorine or —O—CF₃, and xand y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.

Specific highly preferred materials include copolymers oftetrafluoroethylene with 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxolehaving the structure:

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.

Such materials are available commercially from Solvay Solexis, ofThorofare, N.J., under the trade name Hyflon® AD. Different grades areavailable varying in proportions of the dioxole and tetrafluoroethyleneunits, with fluorine:carbon ratios of between 1.5 and 2, depending onthe mix of repeat units. For example, Hyflon® AD60 contains a 60:40ratio of dioxole to tetrafluoroethylene units; Hyflon® AD80 contains an80:20 ratio of dioxole to tetrafluoroethylene units.

Yet other preferred materials have the structure

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1. Such materials areavailable commercially from DuPont Fluoroproducts of Wilmington, Del.under the trade name Teflon® AF. These materials are also available indifferent grades of different glass transition temperature. Teflon®AF1600 is our most preferred grade.

The preparation of composite membranes for gas and liquid separations iswell known in the art, and the membrane may be made by any convenienttechnique. Typically, the microporous support membrane is cast fromsolution onto a removable or non-removable backing, and the selectivelayers are solution coated onto the support. As mentioned above, it ispreferred that the support membrane have an asymmetric structure, withmuch finer, smaller pores in the skin layer to facilitate coating. Suchmembranes may be made by the Loeb-Sourirajan process.

The hydrophilic selective layer is positioned between the supportmembrane and the top selective layer. The hydrophilic layer may becontiguous with the support membrane. In this case, the hydrophiliclayer is usually deposited directly on the support surface by solutioncoating, followed by curing, cross-linking or any other post-depositiontreatment that may be needed. Such steps are familiar to those of skillin the art.

As a less preferred alternative, the support membrane may be cast as anintegral asymmetric membrane from a suitable hydrophilic polymer, thecasting recipe and technique being such that the skin layer of theasymmetric membrane is sufficiently dense, and hence selective, to serveas the hydrophilic layer. Membranes having a cellulose triacetatehydrophilic selective layer can be made in this way, for example.

Instead of the support and hydrophilic layers being contiguous, a gutterlayer may optionally be used between the support membrane and thehydrophilic selective layer, for example to smooth the support surfaceand channel fluid to the support membrane pores. In this case, thesupport membrane is first coated with the gutter layer, then with thehydrophilic layer.

The dioxole-based selective layer is applied as the top selective layer,usually directly onto the hydrophilic layer by solution coating.Optionally a sealing layer may be applied on top of the dioxole-basedlayer to protect the membrane. The use of highly permeable polymers assealing or gutter layers is known in the art.

The membranes may be made in the form of flat sheets or hollow fibers,for example, and formed into membrane modules of any convenient type. Weprefer to use flat sheet membranes assembled into spiral-wound modules.

The hydrophilic layer is shielded from direct contact with the feedfluid by the dioxole-based top selective layer. We have discovered thatthis prevents the hydrophilic layer from excessive swelling anddegradation in the presence of liquids or vapors of high waterconcentration. As a result the processes of the invention provide higherselectivity under certain operating conditions than prior art processesusing membranes with only a hydrophilic selective layer.

As a guideline, the membranes should preferably provide a selectivity ofat least about 50 and more preferably at least about 100, when testedwith a 50/50 ethanol/water mixture at 75° C.

We have found, very surprisingly, that the membranes of the inventionoffer higher selectivity, under conditions where they are exposed to ahigh water concentration in the feed, than can be achieved either by amembrane having only a hydrophilic selective layer or a membrane havingonly a dioxole-based selective layer under the same set of operatingconditions. Comparative test results demonstrating this unexpectedphenomenon with feed solutions containing 20 wt % water or more, andcarried out at the high temperature of 75° C. are given in Example 7below.

The thickness of each of the selective layers independently shouldgenerally be no thicker than 10 μm, and preferably no thicker than 5 μm.In particular, it is preferred that the dioxole-based layer be verythin, such as less than 2 μm, as the dioxole is the less permeablepolymer, and an overly thick layer will reduce the permeance of themembrane to an undesirably low level. Most preferably, the dioxole-basedselective layer thickness should be in the range 0.1-1 μm.

Preferably, the finished membrane provides a water permeance of at leastabout 500 gpu, and most preferably at least about 1,000 gpu, coupledwith a water/organic compound selectivity of at least about 100, when inoperation in the processes of the invention.

The separation factor provided by the process may be higher or lowerthan the membrane selectivity, depending on the relative volatilities ofthe organic component and water.

Returning to FIG. 1, feed stream 107 is passed across feed side 105 ofwater-selective membrane 101. The feed stream is separated into residuestream, 108, which is withdrawn from the feed side as a water-depletedresidue, and permeate stream, 109, which is withdrawn from the permeateside, 106, as a water-enriched permeate, 109.

The driving force for transmembrane permeation of water is thedifference between the water vapor pressure on the feed and permeatesides. In other words, the vapor pressure of water on the feed side ishigher than the vapor pressure on the permeate side. This pressuredifference can be generated in any convenient manner, such as by heatingor compressing the feed stream, by maintaining the permeate side undervacuum, or by a combination of these methods.

The preferred method of generating driving force depends to some extenton whether the process is to be performed in pervaporation or vaporseparation mode. In pervaporation mode, the feed is in the liquid phase,and the pressure on the permeate side is such that the permeating wateris in the gas phase as it emerges from the membrane. In vapor permeationmode, the feed, residue and permeate streams are all vapors as theyenter and leave the membrane unit.

A basic representative embodiment of the invention in pervaporation modeis shown in FIG. 2. In this embodiment, it is assumed that thetransmembrane driving force is created by heating the feed solution andby condensing the permeate vapor. Other methods of providing the drivingforce, such as by using a vacuum pump on the permeate side, couldoptionally be used.

Referring to this figure, liquid feed solution, 204, is heated in step,205, and enters membrane unit or step, 200, as heated feed solution,206. The membrane unit contains water-selective composite membrane, 201,of the composite type described above, having feed side, 202, andpermeate side, 203. Water preferentially permeates the membrane andemerges from the permeate side as permeate vapor stream, 208. Thisstream is passed into condenser or condensation step, 209, and iswithdrawn as water-rich condensate stream, 210. Condensation of thepermeate reduces the vapor pressure in the permeate lines, therebyexposing the permeate side of the membrane to a partial vacuum andincreasing the transmembrane driving force. The dehydrated residuesolution is withdrawn as stream 207 from the feed side.

A basic representative embodiment of the invention in vapor separationmode is shown in FIG. 3. In this embodiment, it is assumed that thetransmembrane driving force is created by compressing the feed vapor andusing a vacuum pump to create a partial vacuum on the permeate side.Other methods of providing the driving force, such as condensing thepermeate vapor, could optionally be used.

Referring to this figure, feed vapor, 304, is compressed in compressoror compression step, 305, and enters membrane unit or step, 300, ascompressed feed vapor, 306. The membrane unit contains water-selectivecomposite membrane, 301, of the composite type described above, havingfeed side, 302, and permeate side, 303. Water vapor preferentiallypermeates the membrane and emerges from the permeate side as permeatevapor stream, 308. This vapor is drawn through vacuum pump, 309, andexhausted as water-rich vapor stream, 310. The dehydrated residue vaporis withdrawn as residue stream, 307, from the feed side.

In both the pervaporation and vapor separation modes of operation,supplying the feed stream to the membrane at elevated temperatureincreases the transmembrane driving force and is preferred. Mostpreferably, the feed stream temperature should be in the range 30-120°C., such as 40° C., 60° C., 75° C. or 100° C., depending on the specificseparation to be performed and other operating parameters. For example,for ethanol/water separations, a typical feed stream temperature mightbe 75° C., 90° C. or 110° C. Temperatures much above 130° C., are notpreferred, and temperatures above about 140° C. should be avoided,because of potential damage to the polymeric membranes or other modulescomponents, such as glues and spacers.

In the simple schematic diagrams of FIGS. 1, 2 and 3, the membraneseparation step is indicated as single box 100, 200 or 300. In eachcase, this step is carried out in a membrane separation unit thatcontains one or more membrane modules. The number of membrane modulesrequired will vary according to the volume flow of the stream to betreated, the composition of the stream, the desired compositions of thepermeate and residue streams, the operating temperature and pressure ofthe system, and the available membrane area per module.

Systems may contain as few as one membrane module or as many as severalhundred or more. The modules may be housed individually in pressurevessels or multiple elements may be mounted together in a sealed housingof appropriate diameter and length. Most preferably, the membranemodules, also known as membrane elements, are housed in a vessel thatprovides heating or reheating within the vessel, as disclosed in U.S.patent application Ser. No. 11/651,303.

Depending on the performance characteristics of the membrane, and theoperating parameters of the system, the process can be designed forvarying levels of separation. A single-stage process in a typicalexample of a feed containing 20 wt % water might remove about 90% ofwater from the feed stream, to yield a residue stream containing 2 wt %water and a permeate stream containing 70 or 80 wt % water. This degreeof separation is adequate for many applications.

If the residue stream requires further purification, it may be passed toa second bank of modules, after reheating if appropriate, for a secondprocessing step. This is generally referred to as a two-step process. Ifthe permeate stream requires further concentration (to recapture avaluable organic that would otherwise be lost, for example) it may bepassed to a second bank of modules for a second-stage treatment. This isgenerally referred to as a two-stage process. Such multi-stage ormulti-step processes, and variants thereof, are familiar to those ofskill in the art, who will appreciate that the process may be configuredin many possible ways, including single-stage, two-step, two-stage, ormore complicated arrays of two or more units in series or cascadearrangements.

The dehydrated organic compound residue stream withdrawn from themembrane separation step is usually the primary product of the processand may pass to any destination. In most dehydration operations, it ispreferred to configure the membrane separation steps to achieve adehydrated product that contains less than 10 wt % water. Depending onthe specific separation, much lower water concentrations in the product,such as less than 5 wt %, less than 1 wt %, or less than 0.5 wt % watermay be required.

The water-rich permeate stream may be sent to any destination. Often,but not necessarily, this stream is simply a waste stream that is cleanenough, as a result of the process of the invention, to discharge to thelocal sewer system. In other circumstances, it may be useful torecirculate this relatively clean water stream within the process, or tothe plant, that produced the feed stream.

The processes of the invention may also include additional separationsteps, carried out, for example, by adsorption, absorption,distillation, condensation or other types of membrane separation, eitherbefore or after the membrane separation process that has been describedabove.

One example of such a process is shown in FIG. 4, which is a schematicdrawing of an embodiment of the invention in which membrane separationis combined with stripping or distillation. The figure is describedbelow as it relates to the removal of water from a stream exiting afermenter used to produce ethanol. This description is not intended tobe limiting—it will be apparent to those of skill in the art that thesame or a similar process could be applied to separate other organiccompounds, of any type and from any source, that have suitablevolatility to be steam stripped preferentially into the overhead vapor.

Referring to FIG. 4, feed stream 400 is a liquid stream from afermentation process, containing ethanol and water, the ethanol beingthe minor component. For example, the ethanol content of the streammight be 3 wt %, 5 wt %, 10 wt % or 12 wt %. In the case that the feedderives directly from a fermenter, the stream may also contain othermaterial that has been carried over from the fermentation step,including solid matter such as cell remnants and insoluble cellulosicmatter, as well as sugars, proteins or the like.

The stream enters stripping column, 401. Such columns are well known andused in many industrial applications. The column may be of any designthat allows contact between liquid and vapor phases in the column, andis preferably a packed or plate column. Pressure and temperatureconditions within the column may be adjusted, as is known in the art, tosuit the specific separation that is being carried out.

In the representative ethanol/water separation example of FIG. 4, thecolumn is often referred to as the beer still. The beer still performs astripping function, the stripping vapor being provided by a reboiler atthe base of the column, but has no rectifier section. This column istypically, but not necessarily, operated under partial vacuumconditions, which can be set by the suction pressure of compressor, 406.If the feed stream is introduced to the column directly from thefermentation step, it will typically be at about 30-40° C.

As the feed liquid descends the column, it is contacted with a risingflow, 402, of stripping vapor generated by reboiler, 404, at the base ofthe column. Ethanol is transferred preferentially over water into therising vapor phase, producing an ethanol-enriched vapor stream, 405,that is withdrawn from the top of the column. In the representativeembodiment shown in FIG. 4, this vapor stream typically contains about50 wt % each of water and ethanol.

Bottoms stream, 403, leaves the bottom of the stripper column, and willusually pass through the reboiler before being withdrawn as dischargestream, 412. This stream contains water and any solids that have beencarried into the column with the feed stream, but typically containsless than 1 wt % ethanol, and preferably 0.1 wt % ethanol or less. Thisstream may be returned to the fermenter, discharged, concentrated torecover the contained solids, or otherwise disposed of as appropriate.

The overhead stream from the column passes through compressor 406,emerging as compressed vapor stream, 407, and enters the membraneseparation unit, 408, which contains water-selective compositemembranes, 409, of the composite type described above. As with FIGS.1-3, the membrane separation unit contains one or multiple membranemodules, arranged in one or multiple steps or stages. For example, theconfiguration may involve two membrane sub-steps, with the residuestream from the first sub-step being passed as feed to the secondsub-step.

Water preferentially permeates the membrane and emerges from thepermeate side as permeate vapor stream, 411. This vapor may be returnedto the column to augment the stripping vapor from the reboiler. Thedehydrated residue vapor is withdrawn as residue stream, 410, from thefeed side.

The invention is expected to be particularly beneficial in theproduction of biofuels, that is fuels produced from biomass of sometype. FIG. 5 illustrates this aspect of the invention, and like FIG. 4is described as it relates to the production of ethanol, although it isnot so limited, and could be used to produce other alcohols or alcoholmixtures, for example.

Referring to FIG. 5, feed biomass, 500, enters fermentation plant orstep, 501. The biomass feedstock may be any biomass that contains afermentable sugar, or that can be processed to produce a fermentablesugar. Examples of biomass that contains fermentable sugars includecorn, sugar cane, beets, fruits and vegetables, wastes from processingfruits and vegetables, and cheese whey. Examples of wastes that can beprocessed to make fermentable sugars include cellulosic materials, suchas grasses, grain stalks, hulls, and other agricultural wastes, andlignocellulosic materials, such as woody materials and wood wastes.

The fermentation itself uses any reaction that can convert a sugar to analcohol and may be carried out in any convenient manner. Numerousfermentation techniques appropriate for use in alcohol production arewell known in the art and described in the literature. The reactor maytake the form of a single vessel, or may be staged, for example toprovide different fermentation conditions in each stage. The reactor maybe operated in any mode, such as batch, fed-batch, semi-continuous orcontinuous mode.

If the source material itself does not contain adequate quantities ofsugar, but may be treated to form sugars, the fermentation step mayinclude sub-steps that convert starch or cellulose to sugar, or thatbreak down lignin and then convert exposed cellulose. These steps may becarried out as pretreatment before the material enters the fermentationvessel, or may be performed simultaneously with the fermentation.

The fermentation step may also include one or more filtration steps, totreat the fermentation broth to recover yeast cells or nutrients, or toremove suspended solids or dissolved salts, for example.

The product broth or solution from the fermentation step, 502, consistsof water, ethanol as a minor component, and typically at least someother dissolved or suspended matter. The ethanol concentration in thisstream is usually, but not necessarily, less than 15 wt % ethanol, suchas 5 wt %, 10 wt % or 12 wt % ethanol. This stream passes to firstseparation step, 503. This step removes some of the water, and raisesthe ethanol concentration by at least about three-fold or five-fold, andpreferably to at least about 50 wt %. The step may be carried out in abeer still, as described above with respect to FIG. 4, or by any otherseparation technique capable of raising the ethanol concentrationsufficiently. In addition to the conventional beer still, anotherpreferred option is to use membrane separation for this step. In thiscase the membranes to be used will preferably be selective in favor ofethanol over water, so as to create an ethanol-enriched permeate streamand a residue stream that is mostly water. The configuration and use ofsuch membranes is taught in U.S. Pat. No. 6,755,975 and in U.S.Published Patent Application 2007/0031954.

This step produces an ethanol-enriched stream, 504, and anethanol-depleted, water-rich stream, 505. Preferably this streamcontains less than 1 wt % ethanol, as can be achieved with either astripping column or a membrane separation unit.

The ethanol-rich stream, which may be in the vapor or liquid phase,passes to second separation step, 506. The goal of this step is todehydrate the ethanol to produce a product that preferably contains atleast 90 wt %, and more preferably higher, such as 95 wt % ethanol orabove. The step may be carried out by any separation technique capableof raising the ethanol concentration to the desired level. In existingprocesses that do not incorporate a membrane separation step, this stepis usually carried out by distillation. In this case, the maximumethanol concentration of the ethanol-rich overhead stream will be theazeotropic concentration, that is, 96 wt % ethanol/4 wt % water. Asanother example, the step may be carried out by dephlegmation, asdescribed in U.S. Pat. No. 6,755,975.

The second separation step produces ethanol-rich stream, 507, andethanol-lean stream, 508. This water-enriched, ethanol-depleted streammay optionally be returned to the inlet of the first separation step.

The ethanol-rich stream, preferably containing at least 90 wt % ethanol,is passed as vapor or liquid to membrane dehydration unit or step, 509.This step uses one or multiple membrane modules, containingwater-selective membranes, 510, of the composite type described above.The modules are arranged in one or multiple steps or stages. Performingthis step as two sub-steps, as shown in FIG. 7, discussed in theExamples section, is often advantageous.

Water preferentially permeates the membranes, to produce a dehydratedethanol product as the residue stream and a water-enriched permeatevapor stream, 512. The permeate vapor stream may optionally berecirculated within the process. The dehydrated ethanol product shouldpreferably contain at least 99 wt % ethanol, and more preferably atleast 99.5 wt % or 99.7 wt % ethanol.

As a less preferred alternative in any embodiment of the invention, adifferent type of polymer material may be used for the second selectivelayer. This material should be capable of deposition as a very thin,dense, non-porous layer onto the hydrophilic selective layer, should beinsoluble in water, and of little or no swellability in water, so as toprovide stable water permeation results at least comparable with thoseshown in FIGS. 8, 9 and 10, and discussed in the Examples section below.The material should also exhibit water/ethanol selectivity of at leastabout 30, and be of sufficiently high permeability that the finishedmembrane has a water permeance of at least about 500 gpu.

One example of such a less preferred material is a perfluorinated cyclicalkyl ether having the structure

where n is a positive integer.This material is available commercially from Asahi Glass Company, ofTokyo, Japan under the trade name Cytop®.

The invention is now further described by the following examples, whichare intended to be illustrative of the invention, but are not intendedto limit the scope or underlying principles in any way.

EXAMPLES Example 1 Membranes

Composite membranes were made. All of them included microporous supportlayers made using standard casting procedures to apply polyvinylidenefluoride (PVDF) solution to polyphenyl sulfide (PPS) paper. One set ofmembranes had a Hyflon® AD60 selective layer applied from a 0.5 wt %solution; the other had a Teflon® AF1600 selective layer applied from a1 wt % solution.

Celfa CMC VP-31 composite membrane was purchased from Folex-Celfa AG,Bahnhofstrasse 6423, Seewen, Switzerland. The membrane is a compositemembrane suitable for pervaporation, with a hydrophilic selective layerof unknown composition.

The Celfa CMC VP-31 has only a hydrophilic selective layer; themembranes with the Hyflon® AD60 and Teflon®AF1600 layers have only adioxole-based selective layer.

Example 2 Water Permeation with Hyflon® AD60 Selective Layer Only

Samples of the Hyflon® AD membranes of Example 1 were cut into stampsand tested in a permeation test-cell apparatus under pervaporationconditions with ethanol/water mixtures containing different amounts ofwater. The permeate pressure was maintained at 2.5 torr and thetemperature of the feed solution was 75° C. The results are shown inTable 1.

TABLE 1 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 4.7 960 15 64 17.81,090 17 64 21.2 1,060 17 63 67.0 1,160 19 61 86.5 1,090 16 68 95.71,370 18 76

As can be seen, the water and ethanol permeances were stable over thetested range, increasing only slightly with increasing waterconcentrations in the feed solution. The selectivity was also maintainedover the range of feed water concentrations, but was only about 60 or70.

Example 3 Water Permeation with Teflon® AF1600 Selective Layer Only

Samples of the Teflon® AF membranes of Example 1 were cut into stampsand tested in a permeation test-cell apparatus under pervaporationconditions with ethanol/water mixtures containing different amounts ofwater. The test conditions were the same as in Example 2. The resultsare shown in Table 2.

TABLE 2 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 3.1 2,660 116 234.7 2,470 108 23 7.2 2,970 110 27 10.9 3,630 121 30 17.8 2,710 100 2767.0 2,940 109 27

As can be seen, this membrane also exhibited good stability underexposure to high concentrations of hot water. The water/ethanolselectivity was considerably lower than for the Hyflon® AD membranes,however.

Example 4 Water Permeation with Celfa CMC VP-31 Membrane (HydrophilicSelective Layer Only)

Samples of the purchased Celfa CMC VP-31 membranes from Example 1 werecut into stamps and tested in a permeation test-cell apparatus underpervaporation conditions with ethanol/water mixtures containingdifferent amounts of water. The test conditions were the same as inExample 2. The results are shown in Table 3.

TABLE 3 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 3.2 3,740 8 470 7.44,310 12 360 10.1 5,870 20 290 15.3 7,370 29 250 22.5 8,650 57 150 30.710,700 147 73

As can be seen, the Celfa membranes exhibited a combination of muchhigher water permeance and much higher water/ethanol selectivity thanthe dioxole-based membranes at low water concentrations. The permeancesto both water and ethanol increased very substantially as the waterconcentration in the feed solution increased, indicating swelling of thehydrophilic membrane in the presence of water. The result was a sharpdecline in membrane selectivity, from over 300 when the waterconcentration was below 10 wt % to below 200 when the waterconcentration was about 20 wt % and below 100 when the waterconcentration was about 30 wt %.

Example 5 Comparison of Hyflon® AD. Teflon® AF and Celfa CMC VP-31Membranes

Results from test-cell experiments of the type reported in Examples 2,3, and 4 were plotted to compare the pervaporation performance of thedifferent membranes. The results are shown in FIG. 8 in the form of aplot of permeate water concentration against feed water concentration.As can be seen, even though this was a simple one-stage experiment, atlow feed water concentrations, the Celfa membranes were able to producea permeate that was mostly water, with only a couple of percent ethanol,an indication of the very high selectivity of the membranes under theseconditions. Under the same conditions, both membranes having onlydioxole-based selective layers performed well, but could not produce apermeate with a water concentration comparable to the Celfa membranes.

At above about 10 wt % water in the feed, the performance of the Celfamembranes began to drop off sharply, and the Celfa membranes performedless well than the Hyflon® AD membranes after the water concentration inthe feed reached about 20 wt % and less well than the Teflon®AFmembranes after the water concentration in the feed reached about 25 wt%.

The Hyflon® AD membranes could produce a permeate containing less than18 wt % ethanol across the entire range of water concentrations.

The experiments were repeated with butanol/water mixtures and similarresults were obtained.

Example 6 Celfa CMC VP 31/Hyflon® AD Membranes in Accordance with theInvention

Celfa CMC VP 31 membranes as purchased were dip-coated in Hyflon® AD60solutions of different polymer concentrations and dried in an oven at60° C. for 10 minutes, to yield membranes of the type shown in FIG. 1,having both a hydrophilic selective layer and a dioxole-based selectivelayer.

The coating solution concentration was varied from 0.25 wt % to 1 wt %.The membranes had dioxole-based selective layers of differentthicknesses, depending on the concentration of Hyflon® AD in the coatingsolution.

Samples of the membranes were cut into stamps and tested in a permeationtest cell apparatus, following the procedure described above for Example2. The results are shown in Tables 4, 5 and 6.

TABLE 4 Membrane made with coating solution concentration of 0.25 wt %Hyflon ®AD60 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 9.78 6,420 14 45021.9 9,910 44 220 50.0 21,140 1,270 17 86.0 27,000 5,460 5

TABLE 5 Membrane made with coating solution concentration of 0.5 wt %Hyflon ®AD60 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 9.78 3,720 8 49021.9 5,330 13 407 50.0 7,110 65 110 86.0 8,640 400 22

TABLE 6 Membrane made with coating solution concentration of 1.0 wt %Hyflon ®AD60 Water Water Ethanol Concentration Permeance PermeanceWater/Ethanol in Feed (wt %) (gpu) (gpu) Selectivity 9.78 2,890 6 49021.9 2,430 6 380 50.0 3,700 23 160 86.0 3,720 91 4

FIG. 9 is a plot comparing the data from Tables 4, 5 and 6 with resultsobtained from Celfa membranes without a Hyflon® AD layer. As can beseen, the membranes with the thinnest Hyflon® AD layer showedessentially the same water permeance as the uncoated Celfa membranes,indicating that the layer was too thin to influence the water permeationproperties. The membrane with the thickest dioxole-based selective layerexhibited the most stable performance over the range of waterconcentrations in terms of water permeance. In other words, the thickestlayer best protected the Celfa membrane from swelling, while stillproviding high permeability to water.

Example 7 Membrane Selectivity Performance Comparison

Samples of three membranes types were prepared:

(i) Celfa CMC VP 31 as purchased;

(ii) 0.5 wt % Hyflon® AD60 selective layer, prepared as in Example 1;

(iii) 0.5 wt % Hyflon® AD60 on purchased Celfa CMC VP 31, prepared as inExample 6.

Only membrane type (iii) was in accordance with the invention.

Samples of the membranes were cut into stamps and tested in a permeationtest cell apparatus, following the procedure described above for Example2. The results are shown in Table 7 and FIG. 10.

TABLE 7 Water Water Ethanol Water/ Concentration Permeance PermeanceEthanol Membrane Type in Feed (wt %) (gpu) (gpu) Selectivity (i)Hydrophilic 7 4310 12 370 selective layer only 22 8,640 57 150 31 10,670147 70 (ii) Dioxole-based 7 770 9 90 selective layer only 28 1,110 18 6067 1,160 19 60 (iii) Hydrophilic 10 3,600 11 320 and dioxole-based 215,650 21 270 selective layers 50 5,890 22 260

As can be seen, the membranes having only a hydrophilic selective layeroutperform the other membranes with respect to water/ethanol selectivityat low water concentrations. The membranes having only a dioxole-basedselective layer exhibit much more stable water/ethanol selectivity, andmatch the selectivity of the hydrophilic membranes when the watercontent of the feed reaches about 30 wt %.

At all water concentrations above about 10 wt %, the membranes havingboth a hydrophilic selective layer and a dioxole-based selective layerexhibit higher selectivity than either the hydrophilic Celfa membrane orthe dioxole-based Hyflon® AD membrane. Furthermore, this selectivityremains reasonably stable and high, at 200 or above, even when the feedsolution contains 80 wt % water. Neither of the other membranes comeclose to this performance, as both have a selectivity less than 100 athigh water concentrations.

Example 8 Comparison of Membranes using Hyflon® AD and Teflon® AF asDioxole-Based Selective Layer

Two sets of membranes with a hydrophilic selective layer and adioxole-based selective layer were made by coating purchased Celfa CMCVP 31 membranes using either a single coating of a solution containing0.5 wt % Teflon® AF or 0.5 wt % Hyflon® AD60.

Samples of the membranes were cut into stamps and tested in a permeationtest cell apparatus, following the procedure described above for Example2. The results are shown as a plot of water permeance of the membranesagainst feed water concentration in FIG. 11. As can be seen, themembranes with the Teflon® AF coating show higher water permeance thanthose with the Hyflon® AD coating over the range of water concentrationsin the feed. For each membrane, the water permeance roughly doubles from10 wt % to 90 wt % feed water concentration.

Example 9 Process Calculations for Stripping/Membrane Hybrid Process

A computer calculation was performed to simulate the performance of aprocess of the type shown in FIG. 4 in separating water from ethanol.The calculation was carried out a modeling program, ChemCad V(ChemStations, Inc., Houston, Tex.), modified with MTR proprietary code.The feed stream to the process was assumed to be a solution of 11.5 wt %ethanol in water; the goal was to produce a dehydrated ethanol streamwith an ethanol concentration of 99.7 wt % ethanol, such as would besuitable as fuel-grade ethanol.

The process uses a stripping step followed by a membrane separationstep, as in FIG. 4. The stripping step was assumed to be performed as ina beer still, with no condensation/rectification for the overhead vaporfrom the column. In this case, the membrane separation step was assumedto be performed in two sub-steps. Each sub-step was assumed to use CelfaCMC VP 31 membranes with an additional selective layer of Hyflon® AD60,prepared as in Example 7. In the alternative, if the feed to the secondsub-step contains a relatively low concentration of water, it ispossible to use a membrane with only the hydrophilic selective layer forthe second sub-step.

The process flow diagram is shown in FIG. 6. Referring to this figure,liquid feed stream, 601, enters stripping column or beer still, 605,which operates at the suction pressure of compressor, 614, that is, halfan atmosphere pressure.

Ethanol-enriched vapor stream, 602, is withdrawn from the top of thecolumn, and water stream, 610, is withdrawn from the bottom, afterpassing through the reboiler (not shown).

The overhead stream from the column passes through compressor, 614, andis cooled, 615, before entering the first membrane separation step, 612,as membrane feed stream, 603. This step uses about 1,600 m² of membranearea to reduce the water content of the process stream to about 10 wt %.Water preferentially permeates the membranes and emerges from thepermeate side as first permeate vapor stream, 608. This stream isreturned to the stripping column. The first dehydrated residue vapor iswithdrawn as residue stream, 604, and passes as feed to the secondmembrane separation step, 613, which uses about 5,000 m² of membranearea.

The residue stream, 607, from this step is the dehydrated ethanolproduct of the process, containing 99.7 wt % ethanol. The secondpermeate stream, 609, is condensed, 616, and pumped by liquid pump, 617,to return to the beer still as stream, 606.

The results of the calculation are shown in Table 8. As can be seen, theprocess produces a high-quality ethanol product and a water stream withvery little ethanol.

TABLE 8 Stream Process Water Ethanol feed stream product 601 602 603 610604 608 607 609 606 Flux (kg/h) 165,000 32,650 32,650 146,111 22,20711,443 18,888 3,318 3,318 Temp. (° C.) 37 70 120 81 116 118 114 30 32Pressure (bar) 1.0 0.5 3.0 0.5 3.0 0.5 3.0 0.1 1.0 Water (wt %) 88.536.3 36.3 99.9 9.9 92.5 0.3 64.7 64.7 Ethanol (wt %) 11.5 63.7 63.7 0.190.1 7.5 99.7 35.3 35.3

Example 10 Process Calculations for Bioethanol Production Process

A computer calculation was performed to simulate the performance of aprocess of the type shown in FIG. 5 to produce ethanol from biomass. Thecalculation was again carried out using ChemCad V. The fermentation stepwas not modeled, but was assumed to produce a solution containing 11.5wt % ethanol in water, as might be produced from conventionalfermentation of corn, for example.

The membrane separation step was assumed to be performed in twosub-steps. Each sub-step was assumed to use Celfa CMC VP 31 membraneswith an additional selective layer of Hyflon® AD60, prepared as inExample 7. In the alternative, the second sub-step, which is exposed toonly a low water concentration in its feed stream, could be carried outusing a membrane having only a hydrophilic selective layer.

The process flow diagram is shown in FIG. 7. Referring to this figure,fermentation step, 711, yields stream, 701, containing 11.5 wt %ethanol. This stream enters beer still, 712, and is separated into waterstream, 702, and overhead vapor stream, 703. The overhead stream fromthe stripper is mixed with return stream, 710, and enters distillationor rectification column, 713, as stream, 704. Both the stripper and therectification column operate at half an atmosphere pressure, created bythe suction of compressor, 717.

The distillation step produces an overhead stream, 716, containing about93 wt % ethanol. Because the membrane separation steps are relied on forthe final purification of the ethanol product, the distillation columnoverhead need not be driven all the way to the azeotrope. The bottomsstream, 706, from this column, like the bottoms stream from thestripper, contains very little ethanol.

The overhead from the distillation column is compressed, 717, condensed,718, and mixed with return stream, 709, to be sent as a feed stream,705, after heating to provide transmembrane driving force (not shown),to the first membrane separation step, 714. This step uses about 1,200m² of membrane area.

Water preferentially permeates the membranes and emerges from thepermeate side as first permeate vapor stream, 710. This stream isrecirculated to be mixed with stream 703 as feed to the rectificationcolumn. The first dehydrated residue vapor is withdrawn as residuestream, 707, and passes as feed to the second membrane separation step,715, which uses about 4,400 m² of membrane area.

The residue stream, 708, from this step is the dehydrated ethanolproduct, containing 99.7 wt % ethanol. The permeate stream, 709, iscondensed, 719, and pumped by liquid pump, 720, to return to the frontof the membrane separation unit.

The results of the calculation are shown in Table 9. Once again, theprocess produces a high-quality ethanol product and a water stream withvery little ethanol.

TABLE 9 Stream Process Water Water Ethanol feed stream stream product701 703 702 716 705 706 707 708 709 710 Flux (kg/h) 165,000 35,273129,726 20,750 22,412 16,387 20,548 18,885 1,663 1,864 Temp (° C.) 37 7381 61 115 81 115 42 110 110 Pressure (bar) 1.0 0.5 0.5 0.5 4.0 0.5 4.04.0 0.1 0.2 Water (wt %) 88.5 46.6 99.9 7.0 9.0 99.9 3.0 0.3 33.7 74.9Ethanol (wt %) 11.5 53.4 0.1 93.0 91.0 0.1 97.0 99.7 66.3 25.1

We claim:
 1. A composite membrane selective between water and organiccompounds, having a feed side and a permeate side, the membranecomprising: (i) a microporous support layer comprising a polymer; (ii) afirst dense selective layer of a hydrophilic polymer; and (iii) a seconddense selective layer of a dioxole-based copolymer having the structure

wherein R₁ and R₂ are fluorine or CF₃, R₃ is fluorine or —O—CF₃, and xand y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1; the first dense selectivelayer being positioned between the microporous support layer and thesecond dense selective layer; the first dense selective layer having ahigher permeance to water, but a lower mechanical stability to waterexposure, than the second dense selective layer; wherein, whenchallenged with a feed solution containing 20 wt % water at a set ofoperating conditions that include a feed solution temperature of 75° C.,the composite membrane has a higher water/organic compound selectivitythan that of either (a) a first membrane having only a hydrophilicpolymer selective layer of the same hydrophilic polymer as the firstdense selective layer, or (b) a second membrane having only adioxole-based polymer selective layer of the same dioxole-based polymeras the second dense selective layer, all as measured at the set ofoperating conditions.
 2. The composite membrane of claim 1, wherein thehydrophilic polymer is chosen from the group consisting of cellulosederivatives and polyvinyl alcohol.
 3. The composite membrane of claim 1,wherein the dioxole-based copolymer has the structure

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.
 4. The composite membraneof claim 1, wherein the dioxole-based copolymer has the structureconsisting of

where x and y represent the relative proportions of the dioxole and thetetrafluoroethylene blocks, such that x+y=1.